Peel all potatoes and slice them as thinly as possible with a sharp knife, keeping white and sweet potatoes in separate piles.

In 10-inch cast iron skillet with oven-safe handle (or you can wrap handle in a double thickness of foil), warm oil over medium heat. Add onion and sauté until tender, about 5 minutes. Transfer to bowl.

Place skillet on bottom rack of oven and bake 25 minutes, or until potatoes are tender when pierced with knife, pressing them down with a spatula occasionally. To serve, loosen edges with spatula and carefully invert potatoes onto warm platter. Garnish with parsley sprigs and cut into wedges.

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